The invention relates to treatment of wastewater from residential, business, industrial, or other sources.
Wastewaters obtained from various sources can contain bulk solid materials (sticks, rags, sand, and other materials that can be screened or filtered therefrom), smaller suspended solid materials that cannot be easily removed by screening or filtration; organic material, such as fine semi-biodegradable organic solids, soluble organic chemicals, pathogenic microorganisms (e.g., bacteria, viruses, spores, cysts, and protists), undesirable nutrients (e.g., nitrogen- or phosphorous-containing compounds that can encourage nuisance growth of algae or weeds at the wastewater discharge site); and inert solid particles (e.g., grit and sand). Discharge of these materials (many of which exert biological oxygen demand {BOD} when discharged) into the environment with the wastewater can have undesirable health and environmental consequences. As a result, wastewater discharges are closely regulated by state and national governments.
Many methods exist for removing undesirable components from wastewater prior to its discharge. Typically, a wastewater treatment method might include one or more treatment steps in which settleable and floatable solids materials are removed, one or more (aerobic and/or anaerobic) microbial degradation steps in which most suspended solids and soluble BOD are removed, and one or more further treatment steps in which wastewater is clarified, disinfected, or further purified.
Solids and suspensions of solids, including inorganic material and organic material, that are removed from wastewater during processing are referred to as sludge. Sludge can be generated at multiple points during wastewater processing, such as at primary and secondary settling steps, and during microbial degradation of wastewater components. Sludges often have strong odor and contain toxic or infectious agents. As a result, wastewater sludge is often treated to degrade odiferous, infectious, or toxic components. Sludge that remains after wastewater treatment must be discarded, for example in a landfill or, less commonly, an incinerator or applied to land (e.g., as a soil fertilizer).
Sludge disposal can represent one of the greatest expenses and most cumbersome steps associated with wastewater treatment. Therefore, reduction of sludge volume can significantly reduce processing costs and improve operating efficiency.
Digesters are processing units that can be used to degrade sludge generated during wastewater processing. The digestion process occurs in the presence of microorganisms and produces carbon dioxide, water, inorganic salts, and (sometimes) methane gas as the microorganisms break down various organic components of the sludge, using them as a source of nutrients and energy for growth. The digestion process results in an increase in the amount of biological material (e.g., microorganisms) but an overall reduction of organic sludge mass. Sludge digesters have been used for many years in wastewater treatment plants in order to reduce the mass of sludge produced, the volume of de-watered sludge that must be disposed, and to reduce the toxicity, infectivity, or both, of sludges produced by wastewater treatment processes. Although there are many configurations, two general types of sludge digesters are used: anaerobic and aerobic digesters.
Anaerobic sludge digesters are large, enclosed vessels that permit conversion of organic materials in sludge to carbon dioxide, water, inorganic salts, and methane. Anaerobic sludge digestion is typically used for concentrated primary sludge having a relatively high BOD strength (e.g., >200,000 milligrams per liter BOD equivalent). This process often involves heating of at least a portion of the digestion apparatus, and can be influenced significantly by fluctuations in operating conditions, input rates, alkalinity, pH, temperature, and populations of organisms in the digester, some of which organisms can interact in counterproductive ways.
Aerobic sludge digesters also convert sludge to carbon dioxide, water, and inorganic salts, but without significant production of methane. Compared with anaerobic digestion units, aerobic digesters are less easily influenced by fluctuations in sludge input, operating conditions, or the specific population of organisms in the digester. However, because aerobic digesters require mixing and aeration of the reactor contents, their construction and their operating and maintenance costs can be higher than anaerobic digesters. Also, the physical layout of aerobic sludge digestion units often limits their use to processes in which a certain volume of wastewater and or sludge can be maintained in the digester.
Only a portion of sludge produced in a wastewater treatment process can be digested, whether by an anaerobic or an aerobic digester. The remaining sludge must be discarded. A significant drawback of aerobic sludge digesters is that the sludge that remains following the digestion process is generally much less concentrated (e.g., less than 2% solids, by dry weight) than sludge produced in an anaerobic digester, which can have concentrations of 6-7% solids, by dry weight. The lower solids content of sludge resulting from aerobic digestion processes increases the disposal costs, increases processing costs necessary to decrease sludge volume, or both. Aerobic digesters are typically limited to digestion of biological sludge from the final clarifiers of a biological treatment system because the high-strength BOD of primary sludge would overwhelm their aeration capacity, leading to problems with foul odors.
A need remains for aerobic sludge digestion apparatus and methods that maintain the speed and ease-of-processing characteristics of existing aerobic degradation processes, but which are able to yield a more concentrated sludge than those existing processes. The invention satisfies this need, at least in part, by providing an improved aeration method capable of handling primary and secondary sludges combined, decreased odor potential, and an easily automated sludge thickening method.